Electrochemical cells, such as batteries, typically employ electrochemical active materials (species that undergo electrochemical oxidation and/or reduction during cell operation) that are inorganic. Common examples include metal/metal cation systems, such as Li/Li+, as well as complex inorganic ions that can intercalate into an electrode upon oxidation or reduction.
Organic active materials are attractive because they are generally lightweight, relatively inexpensive, and amenable to chemical modification/design in order to develop properties suitable for a particular application. However, it has often proven difficult to design organic active materials that have appropriate redox potential, high stability over the course of multiple charge/discharge cycles (in the case of a secondary battery), and other necessary properties.
In addition, organic active materials typically have relatively low energy density (equivalents of charge transfer capability per unit mass or volume) in comparison to inorganic active materials. This is due to the normally large molecular size of organic active materials. One strategy to offset this drawback is to design organic active materials in which the molecule is able to undergo a two-electron (or other multi-electron) reduction/oxidation.
Flow cells or flow batteries are electrochemical cells that do not have solid electrodes but instead have liquid active materials: electrochemical active materials that are liquid in both (or all) reduced and oxidized states. Because there are no solid electrodes to be regenerated via charging, a flow battery can be recharged by draining the discharged liquid active material and refueling with charged liquid active material. This capability to be quickly recharged by refueling makes the use of flow batteries a potentially valuable approach to powering electrical systems that are in near constant use such that extended recharging times would be unacceptable, for example electrically powered municipal transportation vehicles.
However, because flow batteries lack a pool of reduced active material in the form of a solid anode and a sink for oxidized active material in the form of a solid cathode, they typically suffer from low energy density. This low energy density results in a need for frequent refueling and thus largely offsets the value derived from the capability of rapid recharging by refueling.
If a flow cell is to have an organic active material, it is thus particularly important that the organic active material be designed to have a higher than typical energy density.